@Jody_Seal My experience with this is limited to my VW, but what causes the problem is the rerouting of exhaust gases into the intake to burn of any previously unburned fuel as well as crankcase ventilation to the intake manifold. Unlike a MPI engine, there is no fuel injection in the intake ahead of the intake valves. So there is no fuel at all (cheap or otherwise) to act as a solvent to clean the tops of the intake valves. Toyota and now, VW, have started adding a superfluous injector in the intake manifold that squirts fuel into the mix every so often to keep the intake valves clean. So what I think it boils down to is what the requirements are in the marine industry for rerouting exhaust gases and crankcase ventilation to the intake. If it is the same as automotive, save for an injector in the intake manifold, there will be issues.
On VWs, this can get bad by 50K miles. And the proverbial "Italian Tune Up" doesn't seem to bring enough heat into the equation to do much about it. The fix is removing the intake manifold and going after the carbon deposits with a pick and some solvent or, alternatively, walnut blasting. Both run about $700--$800. I have a solvent I spray through my intake (carefully avoiding the MAF) about once per year. My head group was replaced under warranty for unrelated reasons and along with it, I got new intake valves. So I won't know if my spray solution had any effect for another 10K miles or so.